The use of tumour-selective prodrugs (relatively inactive compounds that can be selectively converted to more active compounds in vivo) is a valuable concept in cancer therapy (see for example, Denny, Eur. J: Med. Chem. (2001) 36,577). For example a prodrug may be converted into an anti-tumour agent under the influence of an enzyme that is linkable to a monoclonal antibody that will bind to a tumour associated antigen. The combination of such a prodrug with such an enzyme/monoclonal antibody conjugate represents a very powerful clinical agent. This approach to cancer therapy, often referred to as “antibody directed enzyme prodrug therapy” (ADEPT), is disclosed in international publication WO/1988/007378.
A further therapeutic approach termed “virus-directed enzyme prodrug therapy” (VDEPT) has been proposed as a method for treating tumour cells in patients using prodrugs. Tumour cells are targeted with a viral vector carrying a gene encoding an enzyme capable of activating a prodrug. The gene may be transcriptionally regulated by tissue specific promoter or enhancer sequences. The viral vector enters tumour cells and expresses the enzyme, in order that a prodrug is converted to an active drug within the tumour cells (Huber et al., Proc. Natl. Acad. Sci. USA (1991) 88, 8039). Alternatively, non-viral methods for the delivery of genes have been used. Such methods include calcium phosphate co-precipitation, microinjection, liposomes, direct DNA uptake, and receptor mediated DNA-transfer. These are reviewed in Morgan & French, Annu. Rev. Bioehem., 1993, 62; 191. The term “GDEPT” (gene-directed enzyme prodrug therapy) is used to include both viral and non-viral delivery systems (Denny et al U.S. Pat. No. 6,310,237). One example of a non-viral delivery system being the tumour colonising bacteria Clostridia, utilised in an approach termed clostridia-directed enzyme prodrug therapy (CDEPT).
Many nitroaromatic compounds can be reduced by both mammalian and bacterial flavoprotein enzymes, which effect stepwise addition of up to six electrons. The major enzymatic metabolite is usually the 4-electron reduced species (hydroxylamine). A number of nitrophenyl mustards and nitrophenylaziridines have been reported as prodrugs for use in gene-directed enzyme prodrug therapy (GDEPT) in conjunction with nitroreductase enzymes. In particular, CB 1954 [5-(aziridin-1-yl)-2,4-dinitrobenzamide] (compound 1, scheme 1) is reported to be a substrate for the aerobic bacterial nitroreductase NTR (nfsB gene product) isolated from E. coli (Boland et al., Biochem. Pharmacol. 1991, 41, 867-875; Anlezark et al., Biochem. Pharmacol, 15, 1992, 44, 2289-2295; Parkinson et al., J. Med. Chem. 2000, 43, 3624). This compound has been used as a prodrug in both ADEPT (Knox et al., Biochem. Pharmacol., 1995, 49, 1641-1647) and GDEPT (Bridgewater et al., Eur. J. Cancer, 1995, 31A, 2362-2370; Bailey et al., Gene Ther., 1996, 3, 1143-1150; Bailey and Hart, Gene Ther., 1997, 4, 80-81; Green et al., Cancer Gene Ther., 1997, 4, 229-238) applications, including a clinical trial (Chung-Faye et al., Clin. Cancer Res., 2001, 7, 2662-2668). Similarly, the dinitrophenyl mustard SN 23862 (compound 2, scheme 1) is also a substrate for E. coli NfsB, and shows selective toxicity towards cell lines that express the enzyme. It is activated by nitro group reduction (Palmer et al., J. Med. Chem., 1995, 38, 1229; Kestell et al., Cancer Chemother. Pharmacol., 2000, 46, 365-374). The 4-SO2Me derivative (compound 3, scheme 1) was also a substrate (Atwell et al., Anti-Cancer Drug Des., 1996, 11, 553), as was the dibromo mustard analogue (compound 4, scheme 1) (Atwell et al., J. Med. Chem., 2007, 50, 1197-1212). Prodrugs 1-4 (scheme 1) have poor aqueous solubility. For example, to determine the efficacy of prodrug 4 in xenograft-bearing nude mice, it was administered in either neat DMSO or DMSO/polyethylene glycol/water (Atwell et al., J. Med. Chem., 2007, 50, 1197-1212) resulting in a large variations in maximum tolerated dose.
